Vertebrates show a diverse array of social behaviors associated with territoriality. Field and laboratory experiments indicate that underlying themes—including mechanisms—may exist. For example in birds, extensive evidence over many decades has implicated a role for testosterone in the activation of territorial aggression in reproductive contexts. Territoriality at other times of the year appeared to be independent of gonadal hormone control. One obvious question is—why this diversity of control mechanisms for an apparently similar behavior? Control of testosterone secretion during the breeding season must balance the need to compete with other males (that tends to increase testosterone secretion), and the need to provide parental care (that requires lower testosterone concentrations). Regulation of aggressive behaviors by testosterone in the non-breeding season may incur substantial costs. A series of experiments on the male song sparrow, Melospiza melodia morphna, of western Washington State have revealed possible mechanisms to avoid these costs. Song sparrows are sedentary and defend territories in both breeding and non-breeding seasons. Dominance interactions, territorial aggression and song during the non-breeding season are essentially identical to those during the breeding season. Although in the non-breeding season plasma testosterone and estradiol levels are very low, treatment with an aromatase inhibitor decreases aggression and simultaneous implantation of estradiol restores territorial behavior. These data suggest that the mechanism by which testosterone regulates territorial behavior at the neural level remains intact throughout the year. How the hormonal message to activate such behavior gets to the brain in different season does, however, appear to be different.